2mm Observations and the Search for High-Redshift Dusty Star-forming Galaxies

TL;DR

This study uses deep 2mm ALMA observations to identify high-redshift dusty star-forming galaxies, revealing their number counts, redshift distribution, and star formation rates, and assessing the observational depths needed for complete background light resolution.

Contribution

It demonstrates the effectiveness of 2mm observations in detecting high-redshift dusty galaxies and estimates the depths required to resolve the extragalactic background light fully.

Findings

Good agreement between 2mm counts and previous surveys down to 0.2mJy.

Only a fraction of the background light is resolved at current depths.

Star formation rates exceed 250 solar masses per year at high redshift.

Abstract

Finding high-redshift (z>>4) dusty star-forming galaxies is extremely challenging. It has recently been suggested that millimeter selections may be the best approach, since the negative K-correction makes galaxies at a given far-infrared (FIR) luminosity brighter at z>4 than those at z=2-3. Here we analyze this issue using a deep ALMA 2mm sample obtained by targeting ALMA 870um priors (these priors were the result of targeting SCUBA-2 850um sources) in the GOODS-S. We construct the prior-based 2mm galaxy number counts and compare them with published blank field-based 2mm counts, finding good agreement down to 0.2mJy. Only a fraction of the current 2mm extragalactic background light is resolved, and we estimate what observational depths may be needed to resolve it fully. By complementing the 2mm ALMA data with a deep SCUBA-2 450um sample in the GOODS-S, we exploit the steep gradient with redshift of the 2mm to 450um flux density ratio to estimate redshifts for these galaxies without spectroscopic or robust optical/near-infrared photometric redshifts. Our observations measure galaxies with star formation rates in excess of 250 solar masses per year. For these galaxies, the star formation rate densities fall by a factor of 9 from z=2-3 to z=5-6.